The present invention relates to a heat-sealable paperboard laminate, and more particularly to a non-foil barrier laminate which does not transmit or absorb flavor or odor ingredients of citrus and other juices.
Heat-sealable low density polyethylene (LDPE) is a well known component of the prior art paperboard citrus juice cartons, but it falls short of providing adequate barrier properties to the absorption and/or transmission of citrus juice flavor and odor ingredients. Therefore, additional barrier materials must be added to the laminate structure to achieve adequate barrier properties. In the family of barrier polymers, materials frequently used include: ethylene vinyl alcohol copolymers (EVOH); polyvinylidene chloride and its copolymers (PVDC); polyacrylonitrile and its copolymers (PAN); polyamides (PA): polyethylene terephthalate (PET); polyvinyl chloride (PVC); and polypropylene (PP). Of these materials, EVOH is the preferred material (see article entitled "HIGH BARRIER POLYMERS", by A.L. Blackwell, 1986 Coextrustion Seminar Marriott Hilton Head, Hilton Head, SC, published by TAPPI Press).
EVOH resins are saponified copolymers of ethylene and vinyl acetate. They are characterized as having premier gas barrier properties which resist the permeance of gases such as oxygen and CO.sub.2 for the preservation of food and beverages. EVOH resins also have excellent resistance to oil and organic solvents, thus making them suitable for packaging of oily foods, edible oils, mineral oils agricultural pesticides and organic solvents. They also have excellent fragrance preservation, and packages based on EVOH resins preserve the aroma and flavor of the contents and at the same time prevent entry of fugitive or foreign odors into the package.
Unfortunately EVOH resins have little or no adhesion characteristics to most other polymers except nylon. In addition, EVOH resins are sensitive to the effects of moisture, and the barrier properties are reduced with increasing relative humidity. For this reason, adhesive resins, usually modified polyolefins, have been developed for use in coextrusion applications for incorporating EVOH as a barrier material in barrier laminates. Thus EVOH when incorporated as a barrier component in packaging material is included in a symmetrical sandwich comprising a tie layer/EVOH/tie layer composite, as taught by the prior art, including U.S. Pat. Nos. 4,407,897; 4,789,575; 4,802,943; and, in PACKAGING Magazine, September 1984, pp. 55-60. This combination provides good barrier properties while at the same time protecting the EVOH component from the deteriorating effects of moisture. However, the sandwich construction is an expensive application because of the use of two tie layers. Also in some prior art applications, particularly as shown in the aforementioned '575 patent, where the sandwich tie layer/EVOH/tie layer is applied directly to an uncoated surface of paperboard, it is difficult to achieve proper adhesion of the tie layer to the paperboard surface at a temperature which does not cause degradation and reduced efficiency of the EVOH barrier layer material.
To overcome these and other problems, the present invention omits a tie layer from a conventional EVOH sandwich structure which reduces costs, and adhers the EVOH component of the coextrusion directly to a surface of the paperboard which has previously been coated with LDPE. The ability to heat seal EVOH to LDPE is taught by U.S. Pat. No. 4,702,360. Nevertheless, the present invention is an improvement in the '360 patent because unlike the '360 patent, the EVOH layer in the present invention is not directly exposed to the contents of any package made with the laminate.